In the manufacturing process of semiconductor devices, a polishing apparatus, or more specifically a CMP (Chemical Mechanical Polishing) apparatus, has been widely used for polishing a surface of a wafer. Generally, the CMP apparatus is provided with a film-thickness measuring device for measuring a film thickness of a wafer while the wafer is being polished. The CMP apparatus is configured to measure the film thickness while polishing the wafer, and detect a polishing endpoint of the wafer or control a distribution of remaining film thickness in the wafer surface on the basis of the measured values of the film thickness. The film thickness, which is to be measured by the CMP apparatus, is a thickness of an uppermost film which is an object of polishing. In this specification, “film thickness” shall represent “thickness of a film as an object of polishing” unless otherwise specified.
An optical film-thickness measuring device is known as an example of the film-thickness measuring device. This optical film-thickness measuring device is configured to emit light to a surface of a wafer, receive reflected light from the wafer, and determine a film thickness of the wafer from a spectrum of the reflected light. The spectrum of the reflected light changes in accordance with the film thickness of the wafer. Therefore, the optical film-thickness measuring device is able to determine the film thickness based on the spectrum.